The present invention relates to a solid state imager, and in particular, to a solid state imager suitable for enhancing its sensitivity.
In the prior art, a two-transistor type MOS imaging device having horizontal and vertical MOS switches associated with each photodiode constituting the solid state imaging device has been known. The imager employing such a two-transistior type MOS imaging device is known as a horizontal read out MOS imager. This horizontal read out MOS imager is described in "Institute of Television Engineers of Japan Technical Report" (TEBS 109-3), Vol. 9, No. 45, ED 938 (February, 1986). A summary of the horizontal read out MOS imager described in the abovementioned document is as follows:
Each of the photodiodes forming approximately 500.times.600 pixels is associated with two switches known as horizontal and vertical transistors, and when light impinges on the photodiode, photoelectric charge is accumulated thereon. This photoelectric charge is read out to a horizontal signal line sequentially line by line in response to horizontal and vertical read out pulses. Then, the vertical switch is closed during each horizontal scanning period, and the read out signal is outputted. The outputted signal is supplied to a processing circuit in a subsequent stage through a pre-amplifier.
In the prior art imager, the drawbacks involved are such that when the illuminance of a subject is equal to 100 luxor less, the picture quality becomes gradually deteriorated as a result of reduced illuminance, and disturbances such as random noise and the like becomes increased. It is described in the aforementioned document that the main source of such random noise is a pre-amplifier of the voltage negative feedback type.
According to another known prior art imager in which the read out of a signal from an imaging device is carried out in a voltage read out mode, a pre-amplifier which has no voltage negative feedback is used. Accordingly, the random noise generated in the pre-amplifier is reduced. However, in this imager, the signal output line is reset every time a signal of one pixel is read out. As a result, noise (reset noise) occurs due to the resetting of the signal output line, and this noise is far greater than the random noise generated in the pre-amplifier. Thus, such a prior art imager has not been suitable for practical use.